*2.2. Overset Meshing and Computational Grid*

For conducting the moving-mesh simulation, the overset and dynamic mesh options were applied simultaneously. The overset mesh method allows multiple disconnected meshes to su fficiently overlap each other. Using the overset mesh method to perform a moving mesh helps achieve more e fficient computation time and prevents re-meshing that otherwise reduces the accuracy by causing the generation of poor cells. Generally, the mesh used for the overset method consists of two parts: a background zone and a separate component zone. Background zones are the mesh of the o ff-body or fluid space. The component zones are meshes that contain the objects of analysis; they require overset boundaries. These meshes must be of high quality and should cover the solution domain [14]. The component zones overlay the background zones near the overset boundaries; near these regions, the background and other component zones unify into one zone. An advantage of overset meshing is that individual parts of the overset mesh are created independently, and hence any zone can be easily replaced without having to recreate the whole geometry [15].

In this study, two mesh zones were independently generated. The background zone is the tube mesh and the component zone is the pod mesh. Figure 2 presents the mesh and the generation of the overset mesh used in this study. The grids were generated on ANSYS ICEM 18.1. Both zones were treated with a hexahedral mesh. A finer mesh is created near the pod and the wall to ensure that the maximum *y*+ is maintained around 0.5, except at the nose of the pod, where the highest *y*+ values reach 1.5. Figure 3 shows the variation of the *y*+ values around the pod surface and the tube wall for the highest pod speed case, in other words, 350 m/s. In this study, the maximum and minimum *y*+ values were observed at the nose and tail of the pod, respectively.

**Figure 2.** Schematic of the overset mesh generation. The number of meshes in this figure is only 1/10 of the number of final meshes. Yellow lines represent the overset boundaries where the pod mesh (**a**) and tube mesh connect (**b**). When two meshes overlap, the redundant mesh of the tube mesh vanishes as shown in (**c**).

**Figure 3.** Variation of *y*+ around the pod surface and tube wall at a pod speed of 350 m/s. The maximum *y*+ was obtained at the nose of the pod.

The number of elements in the tube mesh was fixed at 979,951 cells. For the grid independence test, only the pod mesh was demonstrated by comparing the results of two main objectives of this study, in other words, drag force and pressure obtained with several different grids. The pod speed of 350 m/s and tube pressure of 101.325 Pa (1/1000 atm) were used to evaluate the mesh. Table 1 lists the independence test results of the grid computed from three meshes, in other words, coarse mesh (mesh 1), medium mesh (mesh 2), and fine mesh (mesh 3). The difference between the medium mesh and the fine mesh is 0.02% in total drag and 0.004% in pressure; hence, a fine mesh was unnecessary and mesh 2 was chosen. All simulations in this study were conducted with a mesh composed of 1,684,782 cells.

**Table 1.** Grid independent test: Only the pod zone is demonstrated. Drag force and maximum pressure are used to estimate the grid. *Dp*—pressure drag, *Df*—friction drag, *Dt*—total drag, and *Pmax*—maximum pressure. Mesh 2 is applied in all simulations. Total number of elements in this simulation is 1,684,782.

